It is known to provide hydraulic transport systems for carrying particulate matter over long distances in pipe lines or the like, the solids being generally milled to a given particle size range, suspended or slurried in an hydraulic transport medium, generally water, pumped over relatively long distances through troughs, pipelines and the like, and separated at a given destination from the hydraulic transport medium. Thus it is frequently necessary to dewater a slurry or suspension for the hydraulic transport of solids at this destination, thereby freeing the solids from at least the major part of the entraining water.
It is known, in this connection, to carry out the dewatering with concurrent classification of the solids, e.g. separation of the solids into a coarse fraction and a fine fraction with the fine fraction being transferred to a filter stage.
Such systems have been used effectively for the dewatering and classification of coal particles in coal suspension and coal slurry transport technology.
In these systems, an initial stage separates the coarse fraction from the fine fraction by a sieve, whereupon the fine fraction is dewatered in a vacuum filter or decanting centrifuge while the coarse fraction is dewatered in a sieve centrifuge.
Such processes have the disadvantage that sieve machines operate efficiently only with coarse cuts generally in a particle .size range of 0.5-2.0 mm. This cut cannot be dewatered with complete satisfaction since the subsequent centrifugal operation is most efficient for a cut in the particle size range of 0.1-0.3 mm where the dewatering is even more effective than can be carried out with filters on particles in this range.
Another disadvantage is that sieve machines, like hydrocyclones, can provide a satisfactory classification effect only in the presence of large quantities of water which may not always be available and which, if added, increase the amount of material which must be handled.